With the recent dramatic increases in the cost of heating fuels, particularly oil and gas, there has been increasing use of devices and design arrangements for increasing the efficiency of combustion heating devices such as oil and gas-fired furnaces or boilers.
Such combustion heating devices include an arrangement for exhausting the products of combustion to the exterior of the building by way of an exhaust or stack flue duct in communication with the combustion chamber within which the burners are located. In order to improve the draft of such exhaust ducts, there is normally incorporated a diverter hood which serves to mix room air with the exhaust gases to produce sufficient volume to get proper flow through the exhaust flue ducting. The exhaust flue thus allows the loss of warm air whenever the burner is not in operation by virtue of the rapid cooling of the combustion chamber and the heat exchanger structure, such that the stored heat is lost to the outside. In addition, without the exhaust gases flowing into the exhaust ducting, backdrafts into the building through the draft diverter inlet may occur, as well as a loss of relatively warm room air through the draft diverter opening.
Accordingly, automatic damper devices have been devised and utilized which serve to automatically close the exhaust flue ducting when the burner is not in operation. Such devices are activated with the burner control such as to insure that the burner is only operated when the damper device is opened. Reference is made to U.S. Pat. No. 4,039,123 as typical of these devices.
The incorporation of such an automatic flue damper device presents a safety hazard since in the event of a failure in the system resulting in continuing burner operation with the damper actuator in the closed position, the products of combustion tend to spill into the confined space within the building via the draft diverter opening, presenting a hazard to the occupants of the building. The actuator controls are designed to be fail-safe in that the thermostat signal first causes the actuator device to be moved to the open position and then, upon movement of the damper to the open position, an end switch or other similar device is closed to enable burner operation. This enablement is by activating the main valve typically associated with the combustion device burner allowing the oil or gas to be delivered to the burner. In the event of a component failure, such as the main valve being stuck in the open position or other failure, operation of the burner may continue even though the controls call for the burner to be shut off.
Thus, such devices sometimes include a thermostatically operated safety switch in the exhaust stack or draft diverter to sense the resultant high temperature condition existing in the exhaust ducting.
The arrangement described in U.S. Pat. No. 4,039,123 includes a thermostatic switch to interrupt the energization of the automatic flue damper motor which allows a spring to open the damper. However, this still allows continued operation of the burner, and the stuck main gas valve or other failure would not promptly come to the attention of those operating or observing operation of the furnace.
In addition, the opening of the damper cools the switch upon allowing reenergization of the damper motor such that a cycling of the actuator could take place all while the failure of the main gas valve goes undetected.
Temperature sensors associated with the exhaust flue ducting and hence the draft diverter inlet opening present difficulties even though the diverter opening is theoretically a good location to sense blockage of the flue damper and abnormally high temperatures inasmuch as the spillage of combustion gases provides a rapidly developed high temperature condition.
However, the flow pattern through the diverter opening of such combustion gases due to the relatively low volume of combustion gases relative to the volume of inducted air is such that the flow of exhaust gases will not be evenly distributed across the opening of the draft diverter. That is to say, the high temperature condition may be localized at some portion of the draft diverter openings. Accordingly, a sensor which is discretely located at a point within the opening must be located at the particular region whereat the combustion gases tend to exit. This requires tedious flow testing of the system at installation, requiring a skilled service technician to carry it out.
Another condition which may occur due to power failure or other malfunction is the operation of the burners without the air circulation blower being in operation. This causes overheating of the heat exchanger and excessive furnace temperatures. While combustion chamber temperature conditions have been sensed in prior art safety control arrangements, it is difficult to design sensor components to reliably distinguish between normal and abnormal temperature conditions.
While warm air plenum located sensors have been utilized in the past to detect this condition in a more reliable manner, they have relied on shutdown of the main gas valve to correct the situation. In some cases, the main gas valve failure is the cause of the problem and prevents correction of the condition.
It is therefore an object of the present invention to provide a safety control arrangement for combustion heating devices which insure that burner operation will be discontinued in the event of a component failure or other occurrence which results in burner operation with a blocked flue duct such as automatic flue dampers being in the closed position during burner operation.
It is yet another object of the present invention to provide a safety control incorporating a temperature sensor located in the draft diverter inlet opening which does not require complicated testing to determine the proper sensor location.
It is yet another object of the present invention to provide such a safety control arrangement in which the controls may be readily incorporated in the existing burner control circuits without major modifications such that the costs and difficulty of adding the control circuits to such designs is minimized.
It is a further object of the present invention to provide a safety control arrangement for reliably detecting a combustion chamber overheating condition and causing shutdown of burner operation.